Social taboos have been increasingly recognized for their role in determining human behaviour. Such informal institutions may also, in some instances, guide practices that serve as effective conservation measures. Here we present a case in Banten, Indonesia, where a local taboo has discouraged the collection of two herpetofaunal species, the water monitor lizard Varanus salvator and the reticulated python Python reticulatus, on Tinjil Island, an undeveloped island off the coast of Java. The taboo is not observed in the nearby mainland villages of Muara Dua and Cisiih, where the two species may be harvested for skin or meat, and where the water monitor may also be killed as a pest. Water monitors and reticulated pythons figure prominently in the international reptile leather trade, with skins produced from Indonesia's wild populations representing the highest percentage of total global exports of both lizard and snake skins. The site-specific taboo documented here provides a strong deterrent to collection of these species in a location where they could be subject to illicit harvest as populations in nearby mainland areas decrease. Preliminary evidence also suggests that belief in forest guardian spirits may extend protection to other wildlife species on Tinjil Island.